Although optical communications over glass transmission lines, commonly termed optical fibers, are becoming increasingly important, electrical communications over pairs of wires have been important and will likely remain important for a long period of time. Although the bandwidth of wires is extremely limited as compared to the bandwidth of optical fibers, techniques have been developed which increase the information handling capabilities of wire pairs. One such technique is the Integrated Services Digital Network (ISDN) which uses sophisticated data processing techniques to allow data transmission over wire pairs at rates in excess of 100 kbits/sec. This is a remarkable accomplishment when it is considered that analog voice transmission over wires is limited to approximately 4 kHz; high quality audio equipment accurately reproduces signals to frequencies in excess of 20 kHz. Thus, ISDN offers possibilities of greatly increasing the capacity of already installed wires at modest additional cost. Access to and from the wires is provided by interfaces or stations which are transceivers; that is, they combine the functions of transmitters and receivers. To insure the ability of all stations connected to the wires to communicate with each other, the American National Standards Institute (ANSI) has adopted standards for output signals from the transmitter that all interfaces must satisfy. The ANSI standard for the U-interface also defines some aspects of the receiver.
These requirements include efficient conversion of a 2B1Q symbol into four precisely spaced analog levels and subsequent filtering of this four-level analog signal to create an output response which satisfies the ANSI standard. The ANSI standard specifies the accuracy of the four levels indirectly. The standard specifies only the required linearity; the mismatch among levels is a form of nonlinear distortion, and the permitted mismatch is thus specified indirectly. ISDN transceivers require a phase locked loop to perform the timing recovery function. Both analog and digital phase locked loops have been used in the past. For example, Khorramabadi et al. (Khorramabadi), IEEE International Solid States Circuits Conference, Feb. 17, 1989, pp. 256-357, describes a design that uses an analog phase locked loop. Sallaerts, et al. (Sallaerts) IEEE Journal of Solid State Circuits, Vol. 22, December 1987, pp. 1011-1021, and Colbeck, et al. (Colbeck) IEEE Journal of Solid State Circuits, Vol. 24, December 1989, pp. 1614-1624, describes designs that use digital phase locked loops. It was pointed out by D. D. Falconer (Falconer), IEEE Transactions Communications, Vol. COM-33, No. 8, "Timing Jitter Effects on Digital Subscriber Loop Echo Cancellers: Part I--Analysis of the Effect," August 1985, pp. 826-832, that the phase steps introduced by digital phase locked loops severely degrade the echo cancellation and, if not compensated, would cause the receiver to make a large number of decision errors.
Two techniques have been used to cope with this effect. One of them, called "the synchword approach," restricts the instants when phase steps can occur in such a way that they coincide with the reception of the synchronization word (or "synchword") specified by the ANSI standard. Since no data are being received during this period, the degradation of the echo cancellation can be tolerated. Unfortunately, this technique limits the full range of the phase locked loop. Moreover, it is very difficult to confine the degradation of the echo cancellation to the duration of the synchword and, as a result, there is some degradation of the performance of the receiver.
The second technique is the "jitter compensation technique," originally proposed by R. B. P. Carpenter, S. A. Cox, and P. F. Adams: "Jitter Compensation in Echo Cancellers," presented at the IASTED International Symposium for Applied Signal Processing and Digital Filtering, Jun. 19-21, 1985, and by D. G. Messerschmitt: "Asynchronous and Timing Jitter Insensitive Data Echo Cancellation," presented at the IEEE Transactions on Communications, Vol. COM-39, December 1987, pp. 1209-1217. In this technique, an adaptive transversal filter is used to estimate the effect of phase steps on echo cancellation and to generate a compensation signal, which is subtracted from the received signal at the instants when phase steps are generated. This technique has a superior performance and allows a large number of phase steps to be generated without degradation, resulting in a wide pull range for the timing recovery subsystem. However, in order for this technique to perform properly, it is desirable to preserve the time invariance of the echo path. The time invariance requirement for the echo path could easily be violated by an improper design of the transmitter. It is therefore important to provide a transmitter design that preserves this time invariance. Of course, in addition to the technical requirements just mentioned, implementations should use minimal silicon surface area and have minimal power dissipation to enhance prospects of commercial success. Many attempts have been made to implement transmitters satisfying such requirements.
For example, Sallaens describes a transmitter using pulse-density modulation (PDM) and a 1-bit digital to analog (D/A) converter. The pulse-density modulation is not accomplished with sigma-delta modulation but with a sequence stored in ROM (read-only memory). This approach has the drawback that it produces a discontinuity at the transition between two consecutive 2B1Q pulses and that it is sensitive to phase steps. Sigma-delta modulation is a more desirable approach to generating a pulse density modulated signal because it avoids the two drawbacks mentioned above. Colbeck describes a technique for achieving time invariance of the transmitted pulses in spite of phase steps. The technique uses two parallel switched capacitor filters. This technique has the drawback of requiting that the characteristics of the two filters be very closely matched. This requirement is often difficult to satisfy. Khorramabadi describes a transceiver using a second order analog Butterworth filter. The four analog levels must be matched to approximately 14-15 bit relative level accuracy. An alternative approach, using over sampling sigma-delta D/A conversion, could be used to alleviate the linearity or level sensing problem but would require a complicated digital interpolation filter. The filter will require several thousand gates to implement, and the digital modulator will require a datapath at least 20 bits wide.
A transmitter which uses sigma-delta modulation and has low power dissipation, conforms to the ANSI standard, uses a relatively small silicon area, and has a substantially time invariant transmit path would be desirable.